TRIBOLOGICAL AND MECHANICAL BEHAVIOR OF RARE EARTH DOPED COMPOSITE COATING BY THERMAL SPRAYING

Abstract

Rare earth elements (REEs) are known as the “vitamins or nutrients” of metals. The addition of rare earth elements in a limited quantity can enhance the properties of materials. The high pressure-high velocity oxy liquid fuel (HP-HVOLF) also called as high pressure-high velocity oxy fuel (HP-HVOF) process is well-known industry adopted thermal spraying process for developing high melting point powder coatings over various surfaces. This study is mainly focused on effect of adding various rare earth oxides such as erbium oxide (Er2O3)/lanthanum oxide (La2O3) /cerium oxide (CeO2) with a limited quantity in tungsten carbide (WC-10Co-4Cr) powder on surface, mechanical and tribological characterization of coated samples. Each rare earth oxides was added in equal proportions as 0.2, 0.3 and 0.4 wt.% mixed with tungsten carbide powder and deposited on stainless steel using HP-HVOF process. The thickness of deposited coatings varies from 270µm -320µm. The study consists of four sets of experiments as without rare earth oxides as 0 wt.% (Cp1000 - Exp.1) and with each rare earth oxides as 0.2 wt.% ((CpLCE-2) - Exp. 2), 0.3 wt.% ((CpLCE-3) - Exp. 3), 0.4 wt.% ((CpLCE-4) - Exp. 4). All the sets of experiments were characterized using tribological test such as slurry jet erosion test, surface characterization using energy spectroscopy analysis (EDS) scanning electron microscope (SEM), porosity, X-ray diffraction (XRD) analysis, surface roughness, wettability test and mechanical test includes tensile, flexural and hardness test to support the results. Apart from carbide coated samples, the same characterization methods were used for the substrate as SS410. The slurry jet erosion test at different impingement angles was performed on the substrate as well as on all the coated samples. The minimum weight loss has observed as 20.273 mg, 16.658 mg, 19.002 mg, 13.011 mg and 15.183 mg at 15º impingement angle for uncoated (substrate), Cp1000 –Exp.1, (CpLCE-2) - Exp. 2, (CpLCE-3) - Exp. 3, and (CpLCE-4) - Exp. 4 respectively. The eroded surface micrographs were observed using Scanning Electron Microscope at higher magnification. The examined micrographs demonstrated the presence of several wear mechanisms, including micro cutting, micro indentation, ploughing, and crater wear. By the addition of each rare earth vi oxides up to 0.3 wt.% in (CpLCE-3) - Exp. 3 showed the least weight loss due to its high hardness value among all sets of experiments. The EDS (Energy Dispersive Spectroscopy) has recognized the occurrence of different elements on the surface together with rare earth. Moreover, its compounds such as cobalt-tungsten bimetallic carbide (Co3W3C) and tungsten semi-carbide (W2C) are identified using X ray diffraction (XRD) analysis on coated samples. The porosity level of all the coated samples except (CpLCE-2) - Exp. 2 is less than 1%. Moreover the porosity level of (CpLCE-2) - Exp. 2 is ≥1 to ≤ 2%. This leads to higher value of surface roughness. The value of surface roughness is found to be comparable and less for the (CpLCE-3) - Exp. 3, and (CpLCE-4) - Exp. 4 due to grain refinement. The value of static water contact angle for all the coated samples is obtained to be greater than 90º. This causes the coated surface to exhibit hydrophobic properties as evaluated by wettability analysis. The wettability characteristics are influenced by the level of surface roughness. It is well known that in general the higher the value of surface roughness turns the higher contact angle from Wenzel and Cassie Baxter's model, The highest average static water contact angle i.e. 134.4º value has observed for (CpLCE-2) - Exp. 2 due to its highest surface roughness value of 7.02 µm. The mechanical tests have conducted to support the results as well as to explore the possible fields of applications. All the mechanical properties obtained from mechanical test are the highest for (CpLCE-4) - Exp. 4 except hardness. The Vickers hardness values of the coatings obtain from (CpLCE-3) - Exp. 3 showed the highest value i.e. HV (0.3) ≈ 1123. The aim of the current study is to minimize the slurry erosion of the hydro turbine blades. The study reveals that the coatings obtain from (CpLCE-3) - Exp. 3 shows higher erosion resistance. Moreover; the mechanical tests explored the different other fields of applications such as tiller blades for agricultural purposes and to support results of tribological tests.